This invention relates to bottom layer anti-reflective coating compositions useful in multilayer photoresist processes. More particularly, it relates to thermosetting anti-reflective compositions having increased plasma etch rate during submicron processing.
Bottom-applied anti-reflective coatings are used extensively in submicron photoresist processes to eliminate the deleterious effects of standing wave formation during photoresist exposure. They are comprised principally of a polymer binder and a light-absorbing compound, or dye, which may be chemically attached to the binder. The coatings are applied onto the substrate and then overcoated with the photoresist. It is important that the anti-reflective coating and the photoresist do not intermix at the interface between the two layers, otherwise photoresist performance and feature quality will be degraded significantly. This requirement to prevent intermixture between the photoresist and the bottom anti-reflective layer has been met in prior art systems by invoking one of two techniques.
In the first method, the polymer binder is selected to have a highly aromatic structure which contains polar linking groups such as amide, imide, amic acid, sulfone, ketone, and urea moieties at regular sites along the polymer main chain. This combination of structural features prevents the anti-reflective coating from dissolving in the liquid photoresist and makes it resistant to intermixing with the coated photoresist layer when the latter is baked during a typical processing cycle. Anti-reflective coating compositions based on this concept are described in the U.S. Patents incorporated herein by reference and numbered as follows: U.S. Pat. No. 4,910,122 to Arnold et al.; U.S. Pat. No. 5,234,990 to Flaim et al.; U.S. Pat. No. 5,294,680 to Knors et al.; U.S. Pat. No. 5,554,485 to Dichiara et al.; U.S. Pat. No. 5,578,676 to Flaim et al.; U.S. Pat. No. 5,607,824 to Fahey et al.; U.S. Pat. No. 5,654,376 to Knors et al.; U.S. Pat. No. 5,674,648 to Brewer et al.; and U.S. Pat. No. 5,693,691 to Flaim et al.
In the second technique, the polymer binder is designed with thermosetting features, :so that after application the coating can be insolubilized by heating. The binder contains inherently crosslinkable functional groups or groups which can be crosslinked by reaction with an added reagent. In most instances, the dye is attached to the binder. Thermosetting anti-reflective coatings are rapidly becoming the preferred compositions for sub-0.35 micron microlithographic applications because of their superior resistance to intermixing with photoresist layers.
Various dye-attached, thermosetting binder chemistries have been developed. They include phenolic binders such as those described in U.S. Pat. No. 5,597,868 to Kunz; acrylic binders such as those described in European Patent Application No. 94305124.3 by Urano et al., European Patent Application No.92118070.9 by Thackeray, U.S. Pat. Nos. 5,652,297 and 5,652,297 to McCulloch et al., and our co-pending U.S. patent application Ser. No. 08/940,169 by Meador et al.; modified epoxy resin binders such as those described in U.S. Pat. No. 5,693,691 to Flaim et al.; aliphatic polyester binders such as those described in our co-pending U.S. patent application Ser. No. 08/954,425 by Shao et al.; polysilane binders such as those described in U.S. Pat. No. 5,401,614 to Dichiara et al.; and vinyl aromatic binders such as those described in U.S. Pat. No. 5,482,817 to Dichiara et al., all of which are incorporated herein by reference.
In sub-0.35 micron processing, the photoresist pattern is transferred into the anti-reflective coating by plasma etching, usually with oxygen as the active etchant. The plasma etching process erodes the photoresist layer at about the same rate as it erodes the anti-reflective coating since both are organic in nature. This leads to two problems: 1) the sidewalls of the photoresist features are etched laterally, causing a negative feature size bias; and 2) the overall thickness of the photoresist is reduced, making it a less effective mask for subsequent plasma etching of the substrate. Such problems render it critical to minimize the degree of thickness required for an effective anti-reflective coating, increase film optical density (i.e., the light-absorbing power per unit coating thickness), and until now it was critical to use low aromatic content in either acrylic or polyester binders. Anti-reflective coatings prepared from these improved binders thermosetting structures exhibit plasma etch rates 1.2-1.8 times that of older-generation anti-reflective coatings prepared from highly aromatic polysulfones.
However, device feature size is rapidly approaching 0.15 microns, meaning any amount of etch biasing will be significant in comparison to the nominal feature size. Furthermore, the photoresists used to print these small feature sizes operate at an exposing wavelength of 193 nm and have poorer plasma etch resistance than the novolac and poly(hydroxystyrene)-based photoresists in widespread use today at higher wavelengths. As a result, severe photoresist thinning can occur during pattern transfer into the anti-reflective coating, even if the latter is very thin in comparison to the photoresist.
Accordingly, there is a need for polymer anti-reflective coatings which exhibit a faster plasma etch rate relative to photoresist layers than prior art compositions, including the low aromatic acrylics and polyesters. Since the polymer binder is usually the major constituent of the anti-reflective layer, its properties largely control the etch rate of the coating. It is known to those skilled in the art (See J. Electrochem. Soc.: Solid State Science & Technology, p. 206, January 1982.) that oxygenated and halogenated polymers etch more rapidly under oxygen plasma conditions than polymers which contain primarily carbon, and especially those polymers which contain carbocyclic aromatic groups such as phenyl and napthyl rings. This property is believed to be responsible for replacing the highly aromatic copolymers, e.g., polysulfones and polyimides, used in older-generation products with less aromatic and more oxygen-rich acrylic and polyester binders.